The invention relates generally to the area of numerical control apparatus. Specifically the invention provides a method and apparatus for initiating the generation of multiple start threads on a workpiece.
Many numerical controls have the capability of controlling a turning machine to cause it to cut threads on a workpiece held in a rotating spindle. A program generates input signals representing the desired thread lead, spindle speed and thread length. The thread is cut by moving a single point tool in an iterative manner over the workpiece, and the depth of cut is increased with each iteration of movement until the desired thread is cut.
The numerical control contains a spindle feedback circuit connected to the spindle for generating a marker or index pulse every spindle revolution in addition to a predetermined number of spindle pulses every revolution. An interpolator circuit is responsive to the spindle pulses and the input signal defining the thread lead in inches per revolution for generating command pulses to a servomechanism controlling the cutting tool. In a normal threading situation, before the threading span is executed, the cutting tool is moved to a position close to the workpiece. After the programmed threading information is read by the control, the marker pulse is used to initiate the threading pass, i.e. the marker pulse is used to gate spindle pulses to the interpolator to initiate the generation of command pulses. After the first threading pass, the cutting tool returns to the initial starting position. However, the cutting tool is moved to a position closer to the center line of the workpiece thereby increasing the depth of cut. Again, the marker pulse is used to gate spindle pulses to the interpolator. Therefore, the marker pulse guarantees that with every thread pass, the cutting tool makes contact with the workpiece at the same relative rotational position.
With the system described above, it may desired to cut multiple start threads. Since the relative rotational position of the workpiece is fixed with regard to the marker pulse, the starting point of the cutting tool has to be changed. For example, if a two-start thread is required, the second thread would be cut by shifting the tool relative to the workpiece an amount equal to one-half the programmed lead. This is a function of the programmer and requires him to keep track of as many start points as there are threading starts. Further, in the case of longer leads, the start point must be shifted a substantial distance. Aside from the inherent loss of efficiency in such an operation, there are many situations where the configuration of the machine or the workpiece prevents such a large shifting in the start point.
The disclosed invention overcomes the above disadvantages; and in its preferred embodiment, permits a programmer to program a large range of multiple start threads via a simple code which does not cause a change in the start point of the tool for different threading starts.